1. Field of the Invention
The subject invention relates to sealing systems, to floating roof tanks for storing petroleum products or other liquids, to improved sealing systems for a floating roof, to curved seal suspension methods, and to primary seal inspection methods in floating roof systems equipped with primary and secondary seals.
2. Disclosure Statement
This disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior-art inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness, and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material though not necessarily being of itself pertinent.
A floating roof storage tank has a clearance or annular space between the floating roof and the wall of the tank to permit practically unrestrained vertical travel of the roof floating on the liquid stored in the tank. Increasing environmental and economic concerns call for a stricter prevention of vapor escape through the floating roof clearance from petroleum products or other volatile liquids stored in the tank.
In practice, several factors promote, rather than prevent, an escape of vapor from the tank. For one thing, the clearance around the floating roof in the storage tank has to be sufficiently wide to accommodate out-of-roundness and other local dimensional variations in the circularity of the floating roof and tank, caused by such factors as fabrication and erection tolerances, uneven foundation settlements, and directional mechanical or thermal loads, such as the loads caused by high winds and diurnal solar exposure. Such exposure, incidentally, also raises the temperature of the stored liquid, thereby promoting an escape of vapor through the annular clearance.
Unidirectional loads, including high winds, also tend to drive the floating roof on top of the stored liquid, thereby subjecting the width of the annular clearance between the floating roof and the tank to wide variations, while exposing the floating roof to the danger of contact and collision with the tank wall.
Further sealing problems are caused by the propensity of stored liquids and their residues to form crusts and other intractable deposits on the inside of the tank wall.
In the face of these countervailing requirements and difficulties, attempts have been made over more than half a century to solve the underlying problems and satisfy the above mentioned increasing environmental and economic needs.
A recent representative of these efforts is U.S. Pat. No. 4,138,032, by John S. McCabe, issued Feb. 6, 1979, for Full Secondary Seal, Wiper Type, for a Floating Roof Tank, and herewith incorporated by reference herein. That patent follows the now classical approach of providing a secondary seal, mounted at the top of the floating roof, in addition to a primary seal extending around the circumference of the floating roof. According to the approach pursued in that patent, flexible resilient elongated stiffeners are embedded in the elastomeric material of a curved wiper type secondary seal.
According to that cited patent, the reinforced elastomeric seal not only flexes in a vertical arc as the floating roof moves toward and away from any adjoining tank wall portion, but that circumferential stretching and contracting of the seal strip at its outside periphery is unrestrained by the stiffeness, thereby permitting the strip to contact and accommodate to the contour and spacing of the tank wall from the roof edge.
As the cited patent points out, the composite sealing strip therein disclosed, after installation on a floating roof, can acquire an upwardly or downwardly curved or bent shape or a partially upwardly and partially downwardly curved or bent shape as shown in FIGS. 1, 3 and 4 of the cited patent. In practice, the above mentioned problem, including a considerably varying width of the clearance between the tank wall and floating roof and the coarseness and contamination of the inside tank wall, tend to cause curved seals of the type just described to flip over or otherwise loose their efficacy, especially when the floating roof reverses its direction of movement in the operation of the storage tank.
In this respect, the secondary seal configuration shown in FIG. 3 of the cited patent would appear to promote a seepage of rain water or other contaminants into the tank between the circumference of the seal and the adjacent typically coarse tank wall portions. The secondary seal configuration shown in FIG. 4 of the cited patent, on the other hand, appears to display a tendency to act in the manner of a gutter for rain water and other contaminants, which might be disgorged into the storage tank if the seal were to reverse its position from the configuration shown in FIG. 4 to the opposite shape shown in FIG. 3 of the cited patent.
Also, a gradual separation of the elastomeric material from the embedded stiffeners has been noted in practice with some curved seals of the embedded stiffener type.
Reference should in this respect also be had to the patents and background cited in different portions of the above mentioned McCabe patent and herewith incorporated by reference herein.
In the curved secondary seal of the above mentioned U.S. Pat. No. 4,138,032, as well as in a similar seal in U.S. Pat. No. 1,698,158 and in the secondary seal of U.S. Pat. No. 3,167,206, internal or external spring fingers or stiffeners extend practically to the outer circumference of the curved seal at the tank wall. A similar principle may be observed with respect to mechanical linkages and mechanisms which in the past have been proposed for the purpose of retaining the seal at its outer circumference at the tank wall.
In particular, as may, for instance, be seen with reference to U.S. Pat. Nos. 1,673,983, 2,318,134, 2,327,083, 2,329,965, 2,329,966, 2,522,245 and Re. 22,169, previously proposed linkages and mechanisms engage or apply forces to the seal at points which are located in such proximity to the tank wall that the distance between the points of engagement or force application and the tank wall is practically zero. Moreover, these previous proposals couple the seal retaining devices or linkage to the hub of rollers riding along the inside wall of the tank, whereby the distance between such coupling and the tank wall remains constant, irrespective of width variations between the floating roof and the tank wall. Those approaches thus fail to accommodate the seal retention at the tank wall to the different needs brought about by different widths of the clearance between the floating and drifting roof and the inside of the tank wall.
A similar deficiency is observable with respect to those previous proposals which employ springs or resilient members for applying the seal or its retainer to the tank wall. In particular, with existing or previously proposed spring-biased mechanisms, the spring force tends to be weakest when the floating roof has drifted farthest from the tank wall, though the probability of vapor leakage from the storage tank and contaminant seepage into the tank through separation of the seal from the tank wall is greatest at that point.
Another system, in which sealing panels are located on inclined radial arms which retain such panels with intermediate lugs, is apparent from French Pat. No. 1.227.238, issued Oct. 16, 1961 to General American Transportation Corporation. It is, however, not seen how that system could solve the above mentioned problems and meet the above mentioned needs.